Embodiments of the invention are directed, in general, to communication systems and, more specifically, to ranging procedures for use in communication systems.
Orthogonal Frequency Division Multiplexing Access (OFDMA) defines an access scheme of a two-dimensional grid that combines Time Division Access (TDM) with Frequency Division Access (FDM). In OFDMA, data symbols are delivered on subcarriers which form subchannels. Depending on system situation, a predetermined number of subcarriers form one subchannel.
Ranging is the term used to describe the procedure for an SS (subscriber station) to join a network or to change BSs (base stations) as a result of a hand-off. Ranging can also be used to update the frequency or timing offset. However, it is expected that we will not use ranging for this purpose and instead rely on updates from channel estimation.
For application of Time Division Duplexing (TDD) to the OFDMA communication system, ranging is required to acquire accurate timing synchronization between the SS and the BS and adjust the reception power of the BS on the uplink. In each OFDMA frame a ranging channel has a plurality of subchannels for transmitting a ranging signal.
The initial ranging is the process of acquiring a correct timing offset between the BS and the SS and initially adjusting a transmit power. Upon power-on, the SS acquires downlink synchronization from a received downlink preamble signal. Then the SS performs the initial ranging with the BS to adjust an uplink time offset and transmit power. The IEEE 802.16d/e communication systems use the OFDM/OFDMA communication scheme. Thus, they perform a ranging procedure by transmitting a randomly selected ranging code on a plurality of subchannels.
The periodic ranging is the process of periodically tracking the uplink timing offset and received signal strength after the initial ranging. The SS randomly selects one of ranging codes allocated for the periodic ranging in the ranging procedure.